1. Field of the Invention
The present invention relates in general to a hydraulic control circuit, more specifically, to a hydraulic control circuit for reducing heat generation and energy losses due to overload upon a hydraulic pump by adjusting switching position of a control valve through feedback control.
2. Description of the Related Art
Heavy construction machinery and equipment such as an excavator uses a hydraulic control circuit for driving a work machine including a boom and an arm. The operation of the work machine is made possible through an actuator such as a hydraulic cylinder. In general, a constant flow of fluid is discharged from a hydraulic pump discharges and the pressurized fluid is supplied to a hydraulic cylinder by a control valve. The pressurized fluid from the hydraulic cylinder is then collected to a reserve tank and used to drive the actuator.
FIG. 1 is a schematic view of a related art hydraulic control circuit. The hydraulic control circuit of this particular example corresponds to a positive hydraulic control system.
According to the positive hydraulic control system, a hydraulic pump 1 discharges a pressurized fluid flow that is proportional to the manipulated variable of a control lever 4, in order to drive a hydraulic cylinder 2. The configuration and operating relation of the related art hydraulic control system will now be explained below.
The pressurized fluid discharged from the hydraulic pump 1 is supplied to the hydraulic cylinder 2 by a control valve 3. The control valve 3 starts operating by the application of a pilot signal pressure that is generated when the control lever 4 is manipulated. Therefore, the pressurized fluid from the hydraulic pump 1 is supplied to the hydraulic cylinder 2 and the pressurized fluid from the hydraulic cylinder 2 is discharged to a reserve tank T.
A controller 5 controls the hydraulic pump 1 to ensure that the hydraulic pump 1 discharges a pressurized fluid flow that is proportional to the manipulated variable of the control lever 4. To this end, a pressure sensor 9 for detecting a pilot signal pressure is installed on a pilot signal line of the control lever 4, and the pilot signal pressure detected by the pressure sensor 9 is inputted to the controller 5 in form of an electrical signal. The electrical signal of the pressure sensor 9 indicates the manipulated variable of the control lever 4. Thus, the controller 5 operates a corresponding flow out of the input signal from the pressure sensor 9, and transfers a control signal to an electronic depressurizing valve 7. The electronic depressurizing valve 7 receives the control signal from the controller 5, and controls a regulator 8 to control an output flow of the hydraulic pump 1. The control of the output flow of the hydraulic pump 1 is performed in proportion to the manipulated variable of the control lever 4.
However, a problem lies with the above-described related art technique. For example, although it is possible to control the manipulated variable of the control lever 4 (i.e., the output flow of the hydraulic pump 1 can be controlled in accordance with the pilot pressure), a stroke of the control valve 3 is not formed proportionally to the pilot pressure. In result, an overload is applied to the hydraulic pump 1.
To control the speed of a work machine in the heavy construction machinery and equipment, the control valve 3 of the hydraulic control system controls the flow of the pressurized fluid. The spool of the control valve 3 has an orifice forming a passage (or flow channel) for the pressurized fluid. When a pilot signal pressure is applied to the control valve 3, the spool moves and the sectional area of the passage is changed. In consequence, the flow of the pressurized fluid passing through the control valve 3 is changed and thus, the speed of a work machine is controlled.
On the other hand, a work machine such as a boom drops due to its own weight, so the control valve 3 controlling the boom uses a meter-out orifice with an extremely small sectional area, functioning as a passage between the hydraulic cylinder and the reserve tank. As such, when the pressurized fluid passes through the narrow meter-out orifice, the flow rate increases. At this time, different pressures are applied to the spool land surface, and this unbalance in the pressure generates a flow force inside the control valve 3.
Particularly, the flow force applied to an axial direction of the spool of the control valve 3 has a great effect on the controllability and the servo system of the control valve 3. When the passage area of the orifice is increased by moving the spool, the flow force is applied in an opposite direction to the motion of the spool. In result, a stick effect, where the spool does not move in proportion to the manipulated variable of the control lever 4, is generated.
FIG. 2 is a graph illustrating a relation between pilot signal pressure of a control lever and passage area of a control valve spool; and FIG. 3 is a graph illustrating a relation between pilot signal pressure of a control lever and output flow of a hydraulic pump.
In FIG. 2, C-T1 diagram shows a change in normal passage area changes with respect to a change in pilot signal pressure of a meter-out orifice having the smallest passage area formed in the control valve; P-N diagram shows an area change of a passage connecting a hydraulic pump and a neutral passage and discharging a pressurized fluid to a reserve tank; and P-C diagram shows an area change of a passage connecting the hydraulic pump and a hydraulic cylinder.
C-T2 diagram shows an area change of an abnormal passage of the meter-out orifice. According to the C-T2 diagram, even though a pilot signal pressure is inputted to the control valve, the spool does not move properly by the flow force, creating a stagnation. As a result, the amount of the change in passage area of the meter-out orifice is extremely small compared with the C-T1 diagram.
As shown in FIG. 3, when the pilot signal pressure increases by greatly manipulating the control lever, the output flow of the hydraulic pump increases a lot. However, if the operation state of the control valve spool corresponds to the C-T2 diagram by the application of flow force, a great flow force is generated when a large amount of pressurized fluid discharged from the hydraulic pump passes through the meter-out orifice. In such case, overload is applied to the hydraulic pump 1, and the spool does not move proportionally to the manipulated variable of the control lever.
FIG. 4 is a graph illustrating a relation between pilot signal pressure of the control lever and stroke of the control valve spool; and FIG. 5 is a graph illustrating a relation between the pilot signal pressure with respect to time and pump pressure.
In FIG. 4, the solid line A represents the normal stroke of the spool of the control valve 3 when the spool moves by the pilot signal pressure, and the dotted line B represents the abnormal stroke of the spool of the control valve 3.
In FIG. 5, the oblique line C represents a change in pump pressure when the control valve is in normal operation; and the dotted line D represents a change in pump pressure when overload is applied to the hydraulic pump.
Referring to FIG. 4, under the normal operation of the control valve 3 the spool moves in proportion to the magnitude of an input pilot signal pressure to the control valve 3. On the other hand, if a great flow force is generated and thus the operation of the control valve 3 is abnormal, the stroke of the spool is not proportional to the magnitude of the pilot signal pressure. For example, as shown in the dotted line B, the spool of the control valve 3 is stationary despite the pilot signal pressure increase, and suddenly starts moving at a certain point.
Therefore, when the control valve 3 is in abnormal operation, overload is applied to the hydraulic pump 1 as shown in the oblique line D of FIG. 5. The overload on the hydraulic pump 1 is the main cause of energy losses. The stationary state of the spool of the control valve 3 despite the pilot signal pressure increase means that the maneuverability of the equipment is markedly deteriorated.
The flow force generated in the control valve 3 not only affects the controllability and the servo system of the control valve 3 but also causes energy losses. Therefore, to secure safety of the control valve 3 and the entire hydraulic control system, it is necessary to overcome the flow force applied to the spool of the control valve 3.
As an attempt to reduce or compensate the magnitude of the flow force, the spool of the control valve 3 and the sleeve were transformed or the structure of the control valve 3 was changed. In practice, however, these techniques were not useful for the control valve 3. In addition, no matter how much the structure of the control valve 3 was changed, the flow force was not removed completely. This made it more difficult to design a highly stable hydraulic control system.